This invention relates to a carrier frequency error detector for use in a coherent demodulator and, more particularly, to a carrier frequency error detector for use in a coherent demodulator for coherently demodulating a received signal to produce a demodulated signal.
In general, a coherent demodulator of the type described is operable in combination with an incoherent detector for incoherently detecting a received signal in a modulated signal which carries a transmission digital data signal. The modulated signal may be, for example, an N-phase PSK (phase-shift keying) modulated signal where N represents a predetermined integer which is not less than two. The predetermined integer N is usually equal to 2.sup.m where m represents a positive integer. The N-phase PSK modulated signal is also called an N-level PSK modulated signal. The N-phase PSK modulated signal is obtained by carrying out N-phase PSK modulation on a transmission carrier signal having a transmission carrier frequency by the transmission digital data signal. In other words, the N-phase PSK modulated signal is obtained by making a phase of the transmission carrier signal to vary as a function of, as a modulating signal, the transmission digital signal digitized by a transmission clock signal having a transmission clock frequency. The transmission digital data signal comprises a plurality of frames each of which includes a transmission unique word signal indicative of a transmission unique word. The modulated signal may be an APSK (amplitude phase-shift keying) modulated signal which is also referred to a QAM (quadrature amplitude modulation) modulated signal.
The incoherent detector is also referred to a plesiochronous detector. The incoherent detector is supplied through a transmission path with the modulated signal. The incoherent detector includes a local oscillator for oscillating a local carrier signal having a local carrier frequency equal to the transmission carrier frequency. The incoherent detector carries out an incoherent detection operation on the modulated signal by using the local carrier signal. The incoherent detector produces an incoherently detected signal indicative of a result of the incoherent detection operation. The incoherently detected signal is supplied to the coherent demodulator as the received signal. It is noted that the local carrier signal is not synchronized in frequency and in phase with the modulated signal. Accordingly, the received signal has a carrier frequency error and an initial phase error with respect to the transmission carrier frequency and a phase of the modulated signal, respectively.
The coherent demodulator comprises a coherent detector, a carrier recovery circuit, and a clock recovery circuit. The coherent detector is supplied with the received signal. In the manner which will later become clear, the coherent detector is also supplied with a reproduced clock signal and a recovered carrier signal from the clock recovery circuit and the carrier recovery circuit, respectively. The coherent detector carries out a coherent detection operation on the received signal on the basis of the reproduced clock signal and the recovered carrier signal to produce a modulated signal having a baseband level. The modulated signal is supplied to the clock recovery circuit and the carrier recovery circuit.
As mentioned above, the received signal is accompanied with the carrier frequency error and the initial phase error. In order to detect the carrier frequency error and the initial phase error in the received signal, the coherent demodulator further comprises a controlling device. The controlling device comprises a frame timing generator, a carrier frequency error detector, and an initial phase error detector. Responsive to the received signal, the frame timing generator generates a frame timing signal indicative of frame timing. Responsive to the received signal, the carrier frequency error detector detects the carrier frequency error in the received signal to produce a carrier frequency error signal indicative of the carrier frequency error. Responsive to the received signal, the initial phase error detector detects the initial phase error in the received signal to produce an initial phase error signal indicative of the initial phase error. The frame timing signal is supplied to the clock recovery circuit while the carrier frequency error signal and the initial phase error signal are supplied to the carrier recovery circuit.
Responsive to the demodulated signal, the clock recovery circuit generates the reproduced clock signal on the basis of the frame timing signal. Responsive to the demodulated signal, the carrier recovery circuit generates the recovered carrier signal on the basis of the carrier frequency error signal and the initial phase error signal.
In the prior art, the carrier frequency error detector comprises a frequency multiplier, a Fourier transformer, and a maximum detector. It will be assumed that the modulated signal is the N-phase PSK modulated signal. The frequency multiplier is supplied with the received signal. The frequency multiplier carries out a frequency multiplier operation on the received signal by a factor N. The frequency multiplier produces a frequency multiplied signal. The frequency multiplied signal is supplied to the Fourier transformer. The Fourier transformer carries out a Fourier transformation operation on the frequency multiplied signal to produce a Fourier transformed signal indicative of bright line spectra. The Fourier transformed signal is supplied to the maximum detector. Responsive to the Fourier transformed signal, the maximum detector detects a peak in the bright line spectra to produce a maximum detected signal as the carrier frequency error signal. At any rate, the conventional carrier frequency error detector detects a maximum power in a frequency range where the carrier frequency error enables to exist. As a result, the conventional carrier frequency error detector is disadvantageous in that there is the possibility of detecting an incorrect carrier frequency error in a case where noise or the like is superimposed on the received signal.